The Sed festival was a ceremony where the king proved his fitness to reign by running round a marked course. There is evidence of this ritual going back to predynastic times. Usually the first Sed festival was held after 30 years, but Akhenaten held his first one much earlier.
A sed festival for Akhenaten was shown on the Temple of the Aten at Karnak, which must predate the switch of focus away from the 'old' Theban religion to the new one based at Akhetaten. This depiction is unusual in that the god itself is seen undertaking a sed festival of its own. This could be a way of showing that the Aten and Akhenaten are one and the same, with the king being god and vice versa.